Your brand new locomotive won't run. If you push it, it starts to go, and then dies at certain areas. You've checked all your wiring, railjoiners and track sections for problems, but everything seems okay. Now what? Clean the rail!
Clean by friction
Some modelers use eraser-like pads (sold at hobby shops), or an abrasive stone, such as Walther's Bright Boy. Just rub it over the rails. You can buy or make these blocks attached to a car that cleans as it runs in a train, making them a necessity for tunnels.
If you've got super-dirty tracks, you can use a fine grit sandpaper (at least 280 grit- 400 is better)- but be careful! Cleaning with sandpaper exposes a new surface of the metal, and it too will just get dirty if not cleaned with regular cleaner. Many modelers use a sanding sponge as an alternative to just paper.
A touchy subject, often I'm asked about using steel wool on rusty rails. And I usually say it's okay, as long as you get rid of the metal particles that result. Don't use steel wool on anything smaller than O (Lionel) tracks. Run a magnet over the areas that you used steel wool on, as the fibers of steel wool will get sucked into your locomotive's motor and damage it quickly. Only use this if you absolutely have to!
I like to use a liquid cleaner made by LifeLike. It comes blue-colored and thin like water, but it stinks like sweaty clothes (just a small compromise). If you like, you can use rubbing alcohol. Some modelers really love denatured alcohol (used as a lacquer thinner and in furniture refinishing), but if you use that, be careful and wear gloves and goggles.
Place a little on a clean rag, and wipe the rail head clean with about 2 or 4 passes. As you move down the line, keep changing spots on the rag. When you're out of wet area on the rag, put on some more cleaner in a different spot.
To finish up the cleaning, I like to put a few drops of Pacer Technologies' Rail Zip on each rail head at various places along the main line; then run the loco around in circles to spread it around. This keeps the rails and wheels clean, and keeps the oxidation from forming. You'll need to repeat application every 3 to 6 months. If you follow their instructions on the bottle, that is, apply Rail-Zip with a rag, let it dry, and buff it off, you might be able to extend that time.
Keep an eye out- while you're doing this, check the rail joiners and turnouts for any signs of damage or wear. Also go over your wiring to make sure it'll last. I also periodically check my freight cars, especially if they tend to derail a lot. Repairs can be made with parts from your hobby shop.